1. Field of the Invention
The present invention generally relates to display devices, and more particularly to a method for driving a display device having LED-based direct-lit backlight module.
2. The Prior Arts
Liquid crystal has already become the mainstream technology for display devices. It is well known that liquid crystal display (LCD) devices are hold-type display devices due to the retardation property of the liquid crystal molecules. Compared to the impulse-type display devices such as cathode ray tube (CRT) devices, the dynamic response (i.e., the display quality of dynamic images) of the LCD devices has been notoriously inferior. This defect of LCD devices therefore has been the major research and development focuses both throughout academic and industrial arenas.
FIG. 1a is a schematic structural diagram showing a conventional LCD device. As illustrated, cold cathode fluorescent lamp (CCFL) tubes are used as light source for a direct-lit backlight module (denoted as CCFL backlight) and the lighting of the CCFL tubes is controlled by a control circuit. The backlight module usually further contains a diffuser disposed between the backlight module and the display panel to process the light of the CCFL tubes into uniform planar light. The display panel has a number of vertical data lines D1, D2, . . . , Dm (only D1 is shown) and a number of horizontal scan lines G1, G2, . . . , Gn (only G1 is shown). The intersection of each scan line and each data line defines a pixel of the display panel (e.g., the pixel P1 at the intersection of the scan line G1 and the data line D1). Each data line is driven by a data driver and each scan line is enabled by a gate driver. The data and gate drivers are in turn controlled by a control board of the LCD device. Usually, the control board contains a timing controller, a Gamma correction circuit, a power circuit, etc.
The illumination of the pixel P1 is achieved by enabling the scan line G1 by a gate driver and, then, applying a driving voltage onto the data line D1 by a data driver. Due to the retardation property of liquid crystal molecules, the grey level of the pixel P1 under the driving voltage does not reach instantly, but gradually approach, a target level corresponding to the driving voltage. Because of such retardation property (or, low response speed), fast-moving dynamic images on LCD devices suffers residuals, blurring, and flickering. To overcome these problems, a number of methods for speeding up the response of LCD devices are disclosed. FIG. 1b is a waveform diagram showing the timing relationship of various control signals of the LCD device of FIG. 1a. In the diagram, it is assumed that column inversion is used by the LCD device to alter the orientation of the liquid crystal molecules (for people skilled in the related arts, column inversion and other similar methods should be quite familiar to them). In addition, the waveform denoted as Vsync is the vertical synchronization signal of the LCD device, the waveforms G1˜Gn are the enable signals to the scan lines whose pulse width is determined by the horizontal synchronization signal Hsync of the LCD device, the waveform D1 is the driving voltage signal applied to the data line D1, the waveform Vlc is the voltage variation of the pixel P1, the waveform BL is the control signal to the backlight module, and the waveform P1 is the brightness (i.e., grey level) variation of the pixel P1.
As shown in FIG. 1b, assuming that the pixel P1 has a target voltage code16 in frame N-1 and a target voltage code200 in frame N (please note that different target voltages imply different target grey levels), the variation of the pixel P1's grey level in frame N would follow the curve denoted as original and approach the target grey level gradually, if no speed-up method is adopted (i.e., the pixel P1 is directly applied with the target voltage in frame N). A conventional speed-up method is to apply a larger overdriving voltage code220 to the pixel P1. The variation of the pixel P1's grey level in frame N therefore would follow the curve denoted as overdriving and approach the target grey level faster. Another conventional speed-up method is to apply a larger overdriving voltage code230 in the first half period of frame N and then the target voltage code200 in the second half period to the pixel P1. In order to apply two different voltages to within a single frame time, this conventional method doubles the frame rate from 60 Hz to 120 Hz and the method is therefore referred to as a double frame rate (DFR) overdriving method. The variation of the pixel P1's grey level in frame N therefore would follow the curve denoted as DFR overdriving and approach the target grey level even faster. Please note that, for the foregoing approaches, the CCFL backlight is always turned on as can be seen from the waveform BL.
The foregoing approaches are indeed effective in speeding up the LCD device. However, as can be seen from FIG. 1b, the behavior of the liquid crystal molecules in approaching their target grey levels, due to their inherent limitation, can only be improved and cannot be completely eliminated. On the other hand, the spent effort inevitably would cause the increase of cost and better improvement implies higher cost. In practice, under economical concerns, these approaches might not be acceptable.
As the CCFLs suffer potential environmental issues from the mercury vapor contained in the lamp tubes, while light emitting diodes (LEDs) have been advanced to provide superior switching speed, lighting efficiency, and cost, LEDs have become the preferred light source for direct-lit backlight module. On the other hand, the development of the backlight modules was mainly focused on how to enhance the uniformity and brightness of the light provided by the backlight module. But recently, as the LED-based, direct lit solution has become the mainstream technology for backlight modules, there are interests in utilizing the fast switching speed of the backlight LEDs to improve the LCD device's dynamic response.